J. Phys. II France
Volume 5, Numéro 4, April 1995
Page(s) 635 - 654
DOI: 10.1051/jp2:1995154
J. Phys. II France 5 (1995) 635-654

Precipitation of Polyelectrolytes in the Presence of Multivalent Salts

J. Wittmer, A. Johner and J.F. Joanny

Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg Cedex, France

(Received 24 October 1994, received in final form 13 December 1994, accepted 16 December 1994)

We present a theory of the complexation of polyanions by divalent cations. The formation of both mono-and dicomplexes is possible. Monocomplexation locally inverts the charge of the polyelectrolyte and transforms it into a polyampholyte. The formation of a dicomplex creates a bridge between charged monomers. The complexation constants for mono and dicomplexation strongly decrease at high ionic strength. If the complexation constants are large enough, the polyelectrolyte precipitates at moderate salt concentrations. The precipitation is driven either by a Debye-Hückel polarisation energy as in polyampholytes (for almost pure monocomplexation) or by the bridging attraction induced by the dicomplexes. The equilibrium constant for the transformation of two monocomplexes into a dicomplex (at vanishing ionic strength) is a key parameter: - when monocomplexation is favored, the polymer is redissolved by strong charge inversion at moderate salt concentrations. Two successive polymer precipitations and solubilisations can be observed as the divalent salt concentration is increased. - when dicomplexation is favored, the precipitation at moderate salt concentration is followed by a redissolution at high ionic strength where complexation becomes unefficient. Only one precipitation is expected.

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